1. Field of the Invention
This invention relates to heat transfer systems generally and, in particular, to a method and apparatus for remotely monitoring the condition of a heat transfer fluid in the liquid line of a heat transfer system, and for controlling operation of the system in response to the results of the remote monitoring.
2. Description of the Prior Art
A heat transfer system usually includes a compressor for compressing a gaseous heat transfer fluid (commonly called a refrigerant) as the gas is received from an evaporator. The compressed gas is then condensed to a liquid by a cooling fluid moving past and in heat exchange relationship with a condenser. The liquid travels through a liquid line to an expansion valve, which converts the high pressure liquid to a low pressure liquid and gas mixture. The mixture travels through the evaporator where most of the liquid turns to a gas in the process of absorbing heat, and then through a suction line to the compressor where the cycle is repeated.
The purpose of the compressor is to raise the pressure of the refrigerant gas from evaporator pressure to condensing pressure. During compression a considerable amount of heat is added to the gas being compressed, causing the gas to be superheated. This heat must be removed and the refrigerant gas condensed to a liquid ready for use by the expansion valve and evaporator.
The capacity of a condenser to remove heat is affected by the temperature and quantity of the cooling fluid passing in heat exchange relationship with the condenser, and the temperature of the refrigerant gas. The capacity of the condenser will increase whenever the temperature difference between the refrigerant gas and the cooling fluid is increased. This temperature difference may be increased by raising the condensing pressure, by lowering the temperature of the cooling fluid, or by increasing the quantity of cooling fluid passed by the condenser in order to maintain a lower average cooling fluid temperature.
Before condensation can begin, the superheated gas must be cooled to the saturation temperature. After reaching saturation temperature further removal of heat will cause the gas to condense. All superheat and latent heat removed from the refrigerant in the condenser is taken away by the condenser cooling fluid. It is generally desirable to remove further heat by cooling the condensed refrigerant liquid to a lower temperature than indicated by the condensing pressure. This additional cooling of the liquid is called subcooling.
The refrigeration effect is the difference in heat content between the liquid at the temperature it leaves the condenser and the heat content of the vapor entering the compressor, and subcooling can enlarge this difference. This can be calculated for each case, but a generalization can be made for air conditioning applications that for each degree of subcooling the system capacity is increased about 0.5 percent, when the subcooling is not from within the refrigeration cycle itself. This increase is the result of the increased refrigerating effect per pound of refrigerant flow.
Subcooling may be accompanied in the condenser, in a subcooler external to the condenser, or in a liquid line/suction line heat exchanger. The liquid/suction heat exchange subcooling may be used to prevent the formation of bubbles of non-condensed refrigerant in the liquid line to obtain maximum, expansion valve capacity. However, this subcooling effect is obtained from within the refrigeration cycle and doesn't directly increase the refrigerating effect per pound of refrigerant flow.
Therefore, it is preferable to obtain subcooling by removing further heat by cooling the condensed liquid in the condenser itself and/or in a subcooler external to the condenser. This is preferably obtained by moving a cooling fluid past and in heat exchange relationship with the condenser and/or the external subcooler.
Subcooling also allows the system designer more latitude in handling liquid risers and even high liquid line pressure drops if the other limitations in sizing piping are observed. However, subcooling can also create some problems, as will be noted later.
In addition to subcooling to obtain an increase in refrigerating effect, it is also desirable to operate the system so that the head pressure is as low as possible to enable more economical compressor operation. This can be accomplished by lowering the temperature of the refrigerant in the liquid line before the expansion valve to the lowest temperature permissible to obtain the lowest head pressure (condenser temperature). Increasing the cooling effect in the condenser by controlling movement of cooling fluid in heat exchange relationship with the condenser and/or subcooling unit will lower refrigerant temperature in the liquid line.
Thus, there are parameters which are goals in operating a heat transfer system at its maximum capacity and efficiency while using the least energy or power possible. First, the lower the head pressure (condenser pressure), the lower the horsepower requirement for the compressor. Secondly, maximizing subcooling increases compressor capacity as a result of the increased refrigeration effect per pound of refrigerant flow.
On the other hand there are constraints in attempting to achieve these goals. While a lower condensing temperature requires less compressor horsepower, there is a minimum head pressure/condensing pressure required for satisfactory operation of the expansion valve. In many systems the minimum head/condensing pressure is equivalent to about a 90 degree condensing temperature.
If the pressure drops too low in the liquid line, the refrigerant liquid will boil. Since there is no source of heat except the liquid refrigerant, just enough will boil or flash into vapor to lower the temperature of the body of liquid. This vapor is known as flash gas or bubbles of non-condensed refrigerant. The formation of these bubbles in the liquid line before the expansion valve is undesirable, because the gas displaces some of the liquid passing through the expansion valve. This reduces the expansion valve capacity and thus the system capacity. Causes of the pressure drop in the liquid line include friction as the liquid moves through the lines, static head in risers of the liquid line (where the pressure at the top of a column of refrigerant is lower than the pressure at the bottom of the column in a riser), or when the components get out of balance with the design because various components have capacity increases and decreases during operation.
Further, in applications such as air conditioning and chillers, the compressor will operate at a lower suction pressure or temperature when the evaporator load is reduced. If the load falls low enough the suction temperature may fall below 32 degrees F. before the balance point is reached. Therefore, the final temperature of the air will be undesirably low causing the moisture condensing on the evaporator to freeze, or air-flow-obstructing frost to form. The ice or frost forming on the evaporator coil will restrict the air flow and aggravate the condition by forcing the suction temperature even lower. On a chiller, the barrel may freeze.
The system designer will select the sizes and capacities of the components of the system, including sizing the piping and determining the height of any risers, so that the system will operate as efficiently as possible with the load that the system is carrying most of the time. however, loads do not remain constant and provision must be made for the variations.
For example, if the heat transfer system is being used in an air conditioning application, problems occur because as the outdoor temperature drops, the average air conditioning load also drops. These problems may be compounded by a constant internal load requiring system operation even when the outdoor temperatures fall to or below freezing. It is helpful to reduce the condenser capacity so that overall system capacity is reduced as outdoor temperature and the load drops.
Various approaches to cure these problems have been mostly directed to controlling operation of individual components. For example, if condenser capacity needs reducing multiple speed condenser fans have been used which are responsive to liquid line temperatures. Multilouvered dampers have also been used to control air flow past condenser coils because this is less expensive than variable speed for motors. Shutters Or dampers are controlled in response to liquid line pressure, which is approximately equal to head pressure.
Other controls include cycling the condenser fans "off" and "on" in response to reaching a minimum head pressure. The system continues to operate, but the efficiency goes down. Some of the fan cycling systems do not have dividers between the multiple fans. This allows air to be pulled backward and the fans that are operating lose their effectiveness. In low ambients, the fans are turned "on" and "off" very rapidly. Some controls flood the condenser with refrigerant to reduce the effective condenser area. These systems require large amounts of refrigerant for their tonnage sizes.
The prior art controls approach operational problems by trying to control individual components or problems, many times at the expense of increased power consumption and/or reduced system efficiency. Therefore, it is proposed to control the system holistically, that is to approach the system control by coordinating the functional relationship of all of the components.
Accordingly, it is an object of this invention to provide an improved heat transfer system.
It is a further object of this invention to provide an improved control system for heat transfer apparatus.
A still further object of this invention is to provide a control system and a method that operates heat transfer apparatus at its maximum efficiency by reducing head pressure and increasing subcooling without allowing non-condensed gas to exist for any extended period of time in the liquid line of the system.
Another object of this invention is to provide an improved method and device for monitoring the condition of a heat transfer fluid in the liquid line.
It is also an object of this invention to provide an improved method for operating a heat transfer system and for monitoring the condition of a heat transfer fluid in the liquid line.
Other objects, advantages and features of this invention will become apparent when the following description is taken in conjunction with the accompanying drawings.